Chromium plating

ABSTRACT

AN AQUEOUS CHROMIUM ELECTROPLATING BATH COMPRISING A SOURCE OF CHROMIUM IONS AND AN EFFECTIVE MICROCRACKING AMOUNT OF A HALO-NOITROBENZOIC ACID OR ITS BATH SOLUBLE SALTS.

United States Patent Ofice 3,795,593. Patented Mar. 5, 1974 3,795,593 CHROMIUM PLATING Henry Brown, Huntington Woods, Mich., and William A. Boycott, Old Castle, Ontario, Canada, assignors to Oxy Metal Finishing Corporation, Warren, Mich. No Drawing. Filed Feb. 21, 1973, Ser. No. 334,311

Int. Cl. C23b /06 US. Cl. 204-51 16 Claims ABSTRACT OF THE DISCLOSURE An aqueous acidic chromium electroplating bath comprising a source of chromium ions and an effective microcracking amount of a halo-nitrobenzoic acid or its bath soluble salts.

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION This invention relates to the electrodeposition of chromium from aqueous acidic hexavalent chromium solu tions. More particularly, it relates to the electrodeposition of chromium with an increased tendency to develop fine craze-cracking over a wide range of cathode current densities, through the use of halo-nitrobenzoic acids dissolved in acidic hexavalent chromium plating baths.

The main object of this invention is to provide a method to obtain microcracked chromium plate over a wide plat ing range, and with a minimum thickness of the chromium. It has now been found that chloro-nitrobenzoic acids, as exemplified by 2-chloro-5-nitrobenzoic acid (Table 1), when added to acidic hexavalent chromium plating baths containing both sulfate and fluoride (or complex fluoride) anions as catalyst make possible the development of extensive microcracking down into the middle and low current density areas in decorative chromium plating.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is directed to the electrodeposi- 'tion of chromium from an aqueous acidic chromium electroplating bath comprising a source of chromium ions and an etfective microcracking amount of a halo-nitrobenzoic acid or its bath soluble salts, such as an alkali metal (such as potassium, sodium and the like), or nickel, and the like. By microcracking is meant that the chromium deposit has from about 300 to about 3000 cracks per linear inch.

The following example will illustrate the improvements which can be obtained in increasing the extent of microcracking in a decorative chromium plate by the use of a chloro-nitrobenzoic acid dissolved in the bath. A chromium plating bath of the composition CrO 200 g./l. (150-250 g./l. H2804 1 g./l.

Na SiF 2 g./l. (Bath A).

Bath temperature 120 F. (100-125 F.).

can be used for the comparison of the results obtained with and without the presence of 0.5 g./l. of 2-chloro-5- nitrobenzoic acid in the bath. An S shaped steel cathode with square sides instead of rounded sides and with a tab left at the top of the S is formed from a 9 inch long coldrolled steel strip that is 1.25 inches wide. It is cleaned and bright nickel plated for 15 minutes using about 40 amps./ sq. ft. on the panel, rinsed and inserted be tween two lead anodes in about three liters of the chromium plating bath kept at about Using 15 amperes on the panel (average of about amps/sq. ft.) and 10 minutes plating time, the microcracking appears only on a narrow section of the high current density areas, that is, near the edges, with some spotty cracking toward the middle current density areas. With the addition of 0.5 g./l. of 2-chloro-5-nitrobenzoic acid, dense microcracking resulted on all but the most deeply recessed areas of the panel. The number of cracks per linear inch measured in various directions on most of the panel ran from about 600 to 1200, with most of the craze-cracking averaging about 800 to 1000 cracks per linear inch. Thermal shock such as a dip in very hot water to 200 F.) for 1 to 2 minutes or the usual final hot water rinse to aid drying, aids rapid development of microcracking. A highly stressed underneath nickel plate was not used, even though this further increases the microcracking tendency and makes possible the use of thinner chromium plate and still obtain extensive microcracking. The underneath bright nickel plate used in the above test had practically zero stress. The Dubpernell test, that is, about a 10 minute plate at low current density from an acid copper bath applied to the chromium plate was used to identify the extent of microcracking. The chromium surface was photographed at 100 x and the extent and number of microcracks per linear inch were determined.

Instead of using sodium silicofiuoride as the entire source of the silicofiuoride anion, the potassium salt or other silicofiuoride salts can be used to furnish a similar concentration of silicofiuoride anion. Also, saturated solutions of slightly soluble silicofiuoride salts can be used, such as those of lanthanum, proaseodymium or neodymium or the mixtures, and the balance of silicofiuoride anion can be made up with small additions of sodium or potassium silicofiuoride. In this respect, saturated concentrations of ceric (or cerous) silicofiuoride salts of limited bath-solubility are excellent, and only small concentrations or sodium or potassium silicofiuoride need to be added for optimum total concentrations (Bath B) of silicofluoride anion with respect to the low concentrations of sulfate anion for attaining a maximum cathode current density range of decorative microcracked chromium when the chloronitrobenzoic acids are present. The 2-chloro-5- nitrobenzoic acid or 2-chloro-4-nitrobenzoic acid should be present in these baths in their optimum concentration of about 0.5 to about 1 gram/ liter to obtain the most extensive and dense microcracking. It is also possible to control the concentration of silicofiuoride anion by using a saturated solution of potassium silicofiuoride and suppressing the ionization of this salt by the use of high concentrations of potassium ions derived from high concentrations of potassium dichromate added in making up the acidic hexavalent chromium plating bath. Instead of silicofiuoride or fluoride anions, it is possible to use other fluoride containing anions such as fluoaluminates, fluotitanates, fluozirconates, fluophosphates, fluoborates, fluoantimonates, etc., to make up the content of fluoride ion similar to that supplied by about 2 g./l. of sodium silicofiuoride when the ratio of chromic acid to sulfate is kept at about 200 to 1. The sulfate anion can also be controlled by using strontium sulfate with additional strontium dichromate added to repress the ionization of the strontium sulfate. If the fluoride or complex fluoride ion is decreased in concentration, then the degree of microcracking is decreased and yet the halo-nitrobenzoic acids are still useful to extend the degree of microcrack-.

ing. If the fluoride or complex fluoride anions are not used and the chromic acid to sulfate ratio decreased to 150 to 1 or to 100 to 1 or 75 to 1, then for decorative chromium plating where the average thickness of chromium is usually kept to about 0.1 mil as a maximum, then the use of the chloro-nitrobenzoic acids do not cause rnicrocracking. However, the chloro-nitrobenzoic acids are still useful for slightly increasing the extent of micro-cracking with thicker non-decorative use of chromium plate, that is, for engineering uses. In the latter uses, as for hard chrome plate of about 0.3 mil to about 20 mils thickness used for wear resistance, micro craze-cracks of about 100 to 200 cracks per linear inch can serve to retain traces of oil and in this way decrease wear in applications involving wear surfaces as on cylinder liners or on the epitrochoidal track of Wankel rotary enigne housing.

TABLE I.-(CHLORO-'NITROBENZOIC ACIDS) 2-chloro-5-nitrobenzoic acid 2-chloro-4-nitrobenzoic acid 2-chloro-3-nitrobenzoic acid 3-chloro-2-nitrobenzoic acid 3-chlor0-4-nitrobenzoic acid 4-chloro-2-nitrobenzoic acid 4-chloro-3-nitrobenzoic acid 5-chloro-2-nitrobenzoic acid 5-chloro-3-nitrobenzoic acid -6-chlorcr2-nitrobenzoic acid 6-chloro-3-nitrobenzoic acid Z-bromo-6-chloro-3-nitrobenzoic acid 4-chloro-3,S-dinitrobenzoic acid 2-chloro-3,S-dinitrobenzoic acid 2,6-dichloro-4-nitrobenzoic acid 2,6-dichloro-3-nitrobenzoic acid 3,5-dichloro-2-nitrobenzoic acid 3,6-dichloro-2-nitrobenzoic acid 4,5-dichloro-2-nitrobenzoic acid 4,6-dichloro-Z-nitrobenzoic acid 2 ,5-dichloro-3-nitrobenzoic acid 2,6-dichloro-3-nitrobenzoic acid The optimum concentration for the best of the above chloro-nitrobenzoic acids, 2-chloro-5-nitrobenzoic acid and 2-chloro-4-nitrobenzoic acid in the decorative microcrackecl chromium plating baths is about 0.5 g.l. (0.1-1 g./1.); for most of the others, higher concentrations such as 1-2 g./l. are necessary. Actually concentrations as high as 10 g./l., in fact up to satuartion concentrations can be used in the bright decorative chromium plating baths. The compounds of Table I, may be added to the baths as the free acids or as salts, such as the sodium or potassium salts. In any case, in the strongly acid bath, the compounds are present predominantly as acids. Technical grades can be used.

Where thick chromium plate is used in engineering ap plications as for hard chrome plating, then concentrations of the compounds of Table I as low as about 0.05 g./1. are helpful for slightly increased microcracking.

Mixtures of the compounds of Table I, can be used (see Bath B). The compounds of Table I, are compatible with the anti-spray surfactants described in U.S. 2,750,334 and also those described in U.S. 3,432,408.

CrO 200 g./l. (150-250 g./l.). H2504 1g./l.

Ce(SiF '(Satd).

Na siF 0.5 g./l. (Bath B).

Bath temp 120 F. (l10-125 F.). 0.25 g./l. 2-chloro-5-nitrobenzoic acid. 0.25 g./l. 2-chloro-4-nitrobenzoic acid.

The compounds of Table I, function effectively in chromium plating baths containing a ratio of about 300/ 1., and 250/1. of chromic acid to sulfate with the same fluoride concentrations shown with Bath (A) or even with slightly higher fluoride concentrations. However, at the chromic acid concentration shown in Bath (A) and at the concentration of fluoride or complex fluoride shown, the optimum ratio of chromic acid to sulfate is around 200/l. When the ratio is made as high as 300/1. in Bath (A), there is a tendency for haziness to occur in the high current density areas Where the plate is the thickest.

Very small concentrations, 0.1 to about 10 mg./l. of selenious oxide or the equivalent concentration of selenious or selenic acid added to baths of type (A) or (B) will help the microcracking, both the density and extent, when only about 0.1 or about 0.2 g./l. of the chloronitrobenzoic acids are used in these baths. When the optimum quantity of about 0.5 g./l. of the chloro-nitrobenzoic acid is present in the baths of type (A) and (B), then as little as 0.1 to about 2 mg./1. of SeO added to the chromium baths like (A) and (B) will make it possible to obtain the very high density of microcracking (about 800 to 1200 microcracks per linear inch) over a wide current density range, from the highest down to very close to the lowest, even with a decrease of about 0.5 g./l. of the sodium silicofluoride concentration. Thus, Bath (B) would become Bath (C) below, which yields decorative chromium plate with a very extensive and high density microcracking, but with a faint bluish haze or cast.

CrO 200 g./l. (150250 g./l.). H2804 1g./l.

Ce(SiF (Satd) (Bath C).

Bath temp. 120 F. (1lO-125 F.). 0.5 g./l. 2-chloro-5-nitrobenzoic acid. 0.l-2 mg./l. SeO.

The blue haze in the chromium plate from Bath (C) is not as intense as when about 5 to 10 mg./l. of Se0 is added to chromium plating baths, and with concentrations of SeO above 10 mg./l. the blue haze in the decorative chromium plate may be sufficiently noticeable that in many cases it may not be acceptable for decorative plat- I ing. The selenium may be added as selenious oxide, selenious or selenic acids or the salts of the acids, or added as sodium or other selenocyanates, or added as a seleno organic compound such as selenourea, N,N-dimethyl selenourea, triphenyl selenium chloride, etc. Telluric acid does not cause the blue haze, but it is very much less effective than selenious oxide or selenious or selenic acid in. causing or aiding microcracking even when used in concentrations as high as 0.5 to even 2 or 5 g./l.

The rate of consumption of the chloro-nitrobenzoic acids, as exemplified by 2-chloro-5-nitrobenzoic acid is about 0.25 g./1. or 1 gram per gallon for amp. hours per gallon.

The reason extensive microcracking of the decorative chromium deposit makes possible greatly improved corrosion protection with nickel-chromium and copper-nickelchromium plate is as follows. With the highly porous chromium plate, due to the extensive microcracking, there results, in the presence of a corrosive environment, a greatly increased number of small anodic areas (compared to the usual case), and this causes a greatly diminished anodic (corrosion) current in the nickel exposed in the microcracks of the chromium, and consequently the rate of pitting penetration through the nickel is very markedly decreased.

If steel panels are plated with a half mil of bright copper and one mil of bright nickel and chromium plated with 0.03 to 0.05 mil of the usual decorative chromium plate, and these panels are compared to similar steel panels plated with the same copper and nickel plate, but with microcracked chromium of the same thickness as the control, the difference in corrosion protection to the steel on exposure in a marine or industrial atmosphere is strikmg.

As already mentioned, about 2 g./l. is the maximum concentration needed for the compounds of Table I,

though even g./l. or even saturation concentrations which are, in general, about g./l. as the free acid, can be used, though such high concentrations are not necessary and are actually wasteful in most cases.

Halogen substituted organic compounds such as chloroacetic acid, trichloroacetic acid, tetrachlorosuccinic acid, chlorophthalic acid (see US. Pats. 3,505,183, 3,282,812) have been used in acidic hexavalent chromium plating baths. These compounds have no appreciable effect on causing increased microcracking of the plate from acidic chromium plating baths containing both sulfate and fluoride type catalysts.

The chlorophthalic acids, and the chloro-sulfobenzoic acids mentioned along with other halo-organic acids in U.S. 3,505,183 were all used in concentrations of to 100 grams per liter in the acidic hexavalent chromium plating baths of high ratio of chromic acid to catalyst content. They were used for the purpose of increasing the covering power of such baths. They do not appreciably increase the microcracking of the chromium plate, and are thus completely unlike the nitro containing compounds of this invention for their effect in the chromium plating baths. Besides the chloro-nitrobenzoic acids of Table I, other bath-soluble chloro-nitro-organic compounds make possible increased microcracking from baths such as those of bath composition similar to Bath (A). For example, Z-chloro-S-nitrobanzenesulfonic acid and 4-chloro-3-nitrobenzenesulfonic acid and other chloro-nitrobenzenesulfonic acids as well as sulfo-chloro-nitroanthraquinones accomplish increased microcracking. For example, with 2- chloro-S-nitrobenzenesulfonic acid concentrations of 0.5 to about 20 grams/liter can be used, with about 1 to 5 grams/liter giving a similar degree of microcracking as 0.5 g./l. of 2-chloro-5-nitrobenzoic acid in a bath similar to Baths (A) and (C) containing about 200 g./l. chromic acid (150 to 250 g./l.). The chloro-nitroalkanes such as 3-chloro-1-nitropropane (B.P. 197 C.), 2-chloro-l-nitroethane, 1-chloro-2-nitropropane, 2-chloro-nitropropane, lchloro l-nitropropane (B.P. 143 C.), l-chloro-l-nitroethane and 2-chloro-1-nitropropane are very effective in concentrations of about 0.1 to about 1 gram/liter which is close to the saturation concentrations in the chromium plating baths. However, the vapors from these chloronitroalkanes are toxic, and even though the 3-chloro-1- nitropropane has the highest boiling point and the least odor, it is considered to be toxic to breathe; and even if they were used in low concentrations such as about 0.1 g./l. and the balance needed for the most extensive microcracking, be made up of 0.3 to 0.4 g./l. of 2-chloro-5- nitrobenzoic acid or 0.5 to 2 g./l. of 2-chloro-5-nitrobenzenesulfonic acid, it would probably be a hazard to use these volatile compounds over a period of time, even with good ventilation.

It should be noted that chloro-nitrobenzaldehydes when added to acidic hexavalent chromium plating baths are oxidized to the corresponding benzoic acids, and therefore can be added to the electroplating bath of this invention. They are expensive compounds however. Thus the best from the standpoint of results and least cost of the nonvolatile chloro-nitrobenzoic acids, is 2-chloro-5-nitrobenzoic acid, with 2-chloro-S-nitrobenzenesulfonic acid the best of the chloro-nitrobenzenesulfonic acids. The 3- chloro-I-nitropropanes is the most elfective of the chloronitropropanes and the least volatile.

It should be emphasized that when the chromium plating thickness is increased above about 0.03 mil to about 0.05 to 0.1 mil in thickness then the degree (the number of cracks per linear inch) and the extent of microcracking into the lower current density areas is increased for baths like A, B, C. For baths like C it is then not necessary to use even a trace of selenious acid to increase the extent of microcracking when the silicofluoride catalyst concentration is only that obtained from saturation concentrations of cerium silicofluorides in the acidic hexavalent chromium plating bath.

What is claimed is:

1. An aqueous acidic chromium electroplating bath comprising a source of chromium ions and an effective microcracking amount of a halo-nitrobenzoic acid or its bath soluble salts.

2. The bath of claim 1, wherein the amount of acid present ranges from about 0.05 gram/liter to saturation.

3. The bath of claim 1, wherein the halo group is chloro.

4. The bath of claim 1, further comprising halonitrobenzene sulfonic acid or its bath soluble salts in an amount ranging from about 0.5 to about 10 grams/liter.

5. The bath of claim 4, wherein the halo group is chloro.

6. The bath of claim 1, further comprising selenium, and present in an amount ranging from about 0.1 to about 10 mg./l. (calculated as selenious dioxide).

7. The bath of claim 1, wherein the acid is 2-chloro- S-nitrobenzoic acid.

8. The bath of claim 1, wherein the acid is 2-chloro- 4-nitrobenzoic acid.

9. A method of depositing chromium comprising passing an electric current from an anode to a cathode through the electrolyte of claim 1 for a period of time sufficient to form a microcracked chromium deposit.

10. The method of claim 9, wherein the amount of acid present ranges from about 0.05 gram/ liter to saturation.

11. The method of claim 9, wherein the halo group is chloro.

12. The method of claim 9, further comprising halonitrobenzene sulfonic acid or its bath soluble salts in an amount ranging from about 0.5 to about 10 grams/liter.

13. The method of claim 12, wherein the halo group is chloro.

' 14. The method of claim 9, further comprising selenium, and present in an amount ranging from about 0.1 to about 10 mg./l. (calculated as selenious dioxide).

15. The method of claim 9, wherein the acid is 2- chloro-S-nitrobenzoic acid.

16. The method of claim 9, wherein the acid is 2- chloro-4-nitrobenzoic acid.

References Cited UNITED STATES PATENTS 3,505,183 4/1970 Seyb et al 20451 GERALD L. KAPLAN, Primary Examiner 

